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Is the gas velocity of a jet engine relative to the engine speed or is it absolute?
Let's say we are testing a jet engine on the ground by keeping it still. We measure the inlet and outlet velocity of the gas at a specific operation point. Let's assume that we attach this engine on a aircraft that is of course moving. In a random time the operation point is the same as before. If we measure the absolute inlet and outlet velocities are the same as before or only the relative ones(aircraft velocity+gas velocity) are the same?
And one more similar question.
In the case of the engine attached to the aircraft all the fuel energy that can be used is transformed into changes of the kinetic energy of both the aircraft and the air passing through the engine.Right?
In the first case of the test, the kinetic energy of the engine isn't changing, so all the used fuel energy is changing the gas kinetic energy, so in this case the difference in the kinetic energy of the gas is greater. Does this makes sense since we talk for the same operation point?
To conclude, is the thrust the same regardless if the jet engine is moving or not?

Seems like you have a lot of questions here... it may help to break them down into bullets/paragraphs, or even separate questions. Also see existing questions, such as thrust vs. speed and exhaust velocity.

There is no such thing as "absolute" speed, except for speed of light, which is not reached here. So I suggest rewording as "aircraft-relative" and "free-stream-relative" (free stream is the air mass not (yet) affected by the aircraft). There can also be "ground-relative", but that is irrelevant, since ground does not come into any significant interaction here. And to be honest, I am not sure which of the speeds you actually meant by "absolute".

Energy is a bit unusual in that it is conserved in all inertial reference frames, but has different values in each. So you can calculate the energies for aircraft in flight relative to free stream or relative to the aircraft and the energy will add up in both, but in very different ways.

The answer is NO, but it is important to understand why. I will explain the conditions, and answer your questions at the end one by one.
First point you are never testing the same engine:

Aircraft manufacturer usually introduces its inlet and its nozzle so both are not usually the same in a test and in the real conditions. (that will mean is partially installed)

You are not including the general effect of the A/C aerodynamics.
You are not including the air bleeding and oil and fuel pump (also, a partially installed engine)

Essentially you are asking about the effects between installed and uninstalled effects on an engine.
Answering your qeustions:
If we measure the absolute inlet and outlet velocities are the same as before or only the relative ones(aircraft velocity+gas velocity) are the same?

This question is not clear as an engine will have different outlet velocities (fan, main cowl...). Taking care about the right reference systems the speed will be of the same order but affected by my previous argument about installed/uninstalled engines

In the case of the engine attached to the aircraft all the fuel energy that can be used is transformed into changes of the kinetic energy of both the aircraft and the air passing through the engine.Right?

Now, some part is used for bleed air, electric systems, fuel and oil pumps. Not all energy is used to cover the drag of the airplane.

In the first case of the test, the kinetic energy of the engine isn't changing, so all the used fuel energy is changing the gas kinetic energy, so in this case the difference in the kinetic energy of the gas is greater. Does this makes sense since we talk for the same operation point?

Given that you establish the reference system fixed to the engine, you will as well not have a difference in speed produced by the engine. Actually you should used that reference system.


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What variables is an engine's AFR based on?
Maybe this is a bit too technical for an aviation forum, but it specifically concerns a flight engine.(Rotax 912 iS) Road vehicle engines will probably have different behavior. As you may know, the 912iS is a fuel injected engine where you can't manually control the mixture.
I have logged some data on a dyno bench, and i can clearly see a relation between RPM and Air Fuel Ratio. The higher the rpm, the leaner the mixture.
Strangely enough, throttle position or MAP value has no clear relation with the AFR, much unlike a car's engine. I thought i'd ask here since you guys may know what variables a flight engine uses to determine the AFR it wants to maintain.
My guess is that the following variables influence the AFR, but i don't know how and how much:
-Throttle (with exception of WOT or near WOT, which causes rich mixture)
-MAP (i'd guess higher MAP makes for a richer mixture, but not)
-Temperature of intake air, and exhaust gas etc.
-Temperature of oil, coolant etc. (i'd guess a richer AFR is maintained during startup=low temps)
-Known volumetric efficiency at certain circumstances
Finally, are there other factors affecting the AFR, like angular motion of the throttle valve? With carbs used in cars you always had an acceleration pump or a damper in case of a constant depression carb.
This is for the engine to attain the desired rpm/speed more rapidly.
Thanks for the help, cheers.
i'd like to hear if there's other forums more focused on technical aspects of flight.

welcome to aviation.se. please note that we are not a discussion forum, we are a Q&A site. it makes quite a difference. Please consider editing your post to contain only one clear major question, currently I have some difficulties understanding what exactly you are interested in.

As the title says, i'm interested in what circumstances an engine bases it's AFR/mixture on. ie. what circumstances affect the AFR/mixture a flight engine wants to maintain, given the engine regulates the AFR/mixture fully automatically.

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How can the F-15 and Su-27 achieve a high top speed without variable wing geometry?
I watched a documentary once that explained, I believe, how the Su-27 and similar aircraft such as the F-15, can achieve a high top speed without variable wing geometry.
How does this work? How do they achieve high performance without swept wings in the F-15, Su-27, and similar aircraft?

Everyone: could we try not to answer in the comments, please? thank you.

They simply have enough thrust to do so with their given wing. There is no rule that high speeds are associated with variable sweep wings or that you need such a wing to go fast. As a matter of fact many of the testbed super sonic aircraft
as well as the really high speed stuff dont have swept wings.
Wing design can effect efficiency (which may or may not be a concern in fighters) and can limit top speeds but that simply means that for a fixed shape wing it must be optimized for a desired speed range.

Another signifficant factor is because the F-15 is an airforce aircraft. So, it doesn’t have to land on a carrier. Hence it can take plenty of runway length to land, which means it can touchdown at a higher speed. So, its wings can be permanently swept at relatively high angle, because there is no need to have heavy wing pivots that enable them to sweep forward and allow lower speed flight for carrier landings.
In comparison, the F-111 had a variable geometry wing so it could fly slower, when the wings were swept forward, because it was intended as a joint aircraft, and so there was going to be a Navy version, that had to be capable of landing (and takeing off!) on carriers.
The F-111 wing sweep is from 16 to 72.5 degrees. In comparison, the F-15 leading edge of 45 deg. The wing angle is asignificant factor that helps define the mach number at which the shock cone from the nose will impinge the wing tips. See below.
If the wing tips are outside the shock cone and in the sonic flow, the handling charteristics, as well as the increased drag, is likely to be very undesirable. The formula linking mach number and shock angle is given here by NASA.

Interesting explanation about lacking need for an airforce aircraft to land at lower speeds, and it surely justifies differences with F-14 for the navy, but in the navy actually flies the F-18 that has fixed wings but less angled... maybe because F-18 is slower than F-14 at top speed?

@LucaDetomi.  Yes, I would agree that because the F-18 has a lower top speed, it doesn’t need variable sweep wings. Variable sweep adds a lot of weight, which is not good when you want a 10 g aircraft for dog fighting. Most aspects of engineering involve a comprise. If you gain somewhere, you are losing somewhere else. Cheers

All sweep does is make the air think the wing's chord to thickness ratio is higher, or you could say thinner in relation to its chord than it really is, for a given wing area.  The benefit is mainly for wings with a fairly high aspect ratio, that is long with a narrow chord.  Fooling the air into thinking the wing is "finer" than it really is delays shock wave formation to a higher speed than if the same wing, with the same chord and length, was straight.
Airplanes like the F-15 with a low aspect ratio, fairly thin wing don't really need a lot of sweep because their airfoils are already really thin relative to their chord.
The F104 is the most extreme example of this.  Ultra low aspect ratio, thin biconvex supersonic airfoil, no sweep at all required because the benefit is negligible.

No, sweep does a lot more. Please ask for "supersonic leading edge suction" to learn what else happens when wings are swept.

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What CPU does the F-16 use?
I've read that the "F-16", without it being specified what version, uses the 1750A CPU, it is on the list here, although without a citation: https://en.wikipedia.org/wiki/MIL-STD-1750A
However, surely this can't be right for more modern F-16's?
I'm curious what CPU the F-16A, C, E, and IN uses.
Perhaps even the more modern F-16's still retain the 1750A for some "legacy" features, but have a more modern "front end" CPU to handle the more modern features?

Not clear what you are asking here. A system of that size could easily have more than one CPU per subsystem. You can safely assume fly-by-wire has 2-3 CPUs different from the auto pilot. 1750A you mentioned was used by flight control and fire control, while the answer below listed MIPS family for mission control. They may co-exist on one aircraft easily.

@user3528438 Good point, I completely overlooked the other subsystems. In my mind, the mission computers constituted the 'brains' of the aircraft, but it's an incomplete picture. I guess I should clarify the answer.

The F-16 was designed in the 1970s, so expect 1970s technology in the oldest of them. Of course many of them will have been upgraded over the 4 decades since they entered service.

airplanes' computer's architecture may be different from your desktop computer. It may be based on specialized chipsets (real time constraints, specialized computation,...) I'm not quite sure you ca n compare it by only focusing on CPUs.

@jwenting: The F-16 received a mid-life update (from Block 50 on, IIRC) because the additional load of terrain avoidance made new processors necessary. First flight was even before the 1750A standard was finished, so a range of processors were/are used.

I think this gets into the common misconception that there is only on CPU for all of the systems in the aircraft. Perhaps if you were looking for the microcontroller used in a specific system, as @aerobot has in their answer, you might get better results.

@PeterKämpf whether an MLU was performed and if so what was included would depend on the operator and the individual airframe.

MIL-STD-1760A is simply the standard that a processor must conform to, and is not an actual model of CPU.
There are actually a lot of independent computers in a military aircraft, each with specific responsibilities. They are usually linked up via networks, typically over MIL-STD-1553 busses for older platforms. The radar has its own processor(s) and architecture(s), as does the flight control module, the fire control module, the navigation system, the munition controllers (sometimes one per weapon station/pylon), the display controller, the mission computer, et cetera. All of these come from different OEMs, and therefore use a variety of hardware. The different subsystems present and their exact functionalities depend on the design and requirements. A subsystem may itself be composed of multiple computers within their own network, independent of the main aircraft networks.
The F-16 uses Raytheon's Modular Mission Computer (MMC) for its mission computer, with the following specs:

The first generation, the MMC3000 relied on MIPS R3500, a 32-bit chip processors with 110,000 transistors and running at 12 MHz. We found this chip on the end of the 1990s HP9000 computers. The computer had 4 MB of memory RAM.
The MMC5000, the second generation of the F-16 computer, still relied on a chip MIPS, a RM5260 who was noticeably faster (with a 133 MHz to 150 MHz clock frequency) and above all it is a 64-bit chip. The memory was more than doubled, with 10 MB available.
The MMC7000 that equips the more modern F-16, including all F-16 Europeans who benefited from the MLU (mid-life Update) modernisation, has always that 10 MB of memory but its RM7000A processor, designed in the early 2000s, works between 300 and 400 MHz.
Source: http://www.4erevolution.com/en/ordinateur-de-bord-f-16/

I highlighted the mission computer because I think of the mission computer/controller/processor (exact terminology depends on the platform) as the brains of a military aircraft. They tend to be the bus controller of the main aircraft network, aggregating inputs from other subsystems and computing/providing information to the pilot to help them carry out their mission.

To the second part of your question, I assume you mean faster or more powerful CPUs when you talk about modern CPUs. Modern here depends on context. CPUs used In aircraft will never need the type of clock speeds or memory capacity you see in consumer computers, for a few reasons. Firstly, there is actually a lot of processing capacity distributed over the various subsystems - the need for a single powerful chip only arises when you demand that a single chip do everything. Secondly, when the requirements of subsystems are as specific as they as in military platforms, the software can be very compact and efficient, unlike consumer software (which require a lot more code for things like architecture adaptors, multi-tasking logic, and thousands of device drivers) - you trade off unnecessary flexibility in return for reliability and efficiency.
The multi-functional displays of military aircraft (and also commercial ones) focus on conveying the necessary information in a clear and concise manner. The rendering requirements are actually quite primitive compared to modern 3D games - it can get by with less processing power than a computer from the 1990s.

EDIT: Elaborated answer, following user3528438's comment.

A quibble.  MIL-STD-1750A is 16-bit instruction set architecture.  MIL-STD-1760 is aircraft/stores interface.

F-16 Fire Control Computer Design Details
F-16A/B aircraft Fire Control Computer (FCC) was procured from Delco Electronics, Goleta, California (a General Motors Division).  Its proprietary instruction set/architecture processor was designated (by DE) as the M362F-2: "M362" was the general design name, "F" for floating point instruction set, and "-2" for the second version of that instruction set.  Other details:

32k core memory in two 16k stacks (supposedly enough memory "for all time").  A note: the "cores" (the individual bit memory elements) were torus-shaped ferrite 13 mils (0.013 inches) in diameter.  The center hole was 7 mils diameter.  Two wires passed through the core.
Clock frequency: 4 MHz (250ns clock period)
TTL integrated circuits, mainly 54LSxx parts family although some 54Sxx used in critical propagation paths
Microprogrammed processor (16 bit word size) on two circuit cards, P1F and P2F
Sixteen 16-bit wide general purpose CPU registers (much like Data General NOVA minicomputers)
CPU included several "macro" instructions implementing floating multiply, floating division, polynomial expansion coefficient generation, square root, arctangent in two quadrants to convert fuel flow meter synchro signals to shaft angle
I/O was general digital discrete, both 5V ("low level") and 26VDC ("high level")
Single dual-redundant Mil-STD-1553 serial data bus
Power consumption: 150W from single phase 115VAC 400Hz aircraft power

See f-16.net/forum/viewtopic.php?t=53181 for more M362F-2 details.

Photo of one side of a P1F CCA from a M362F computer.  This CCA is identical construction with the P1F CCA used in the FCC.

F-16C/D aircraft were initially equipped with the Enhanced Fire Control Computer (EFCC) also supplied by Delco Electronics.  Its processor was designated the M372F-1: "M372" was design name for DE's Mil-STD-1750A compliant processor, "F" for floating point instruction set, and "-1" for that version of the instruction set.  DE's internal name for the computer was "D-cubed" and written as "D3".  "D" stood for "double" three elements of the M362F-2 design: twice the clock speed as the M362F-2, two dual redundant -1553 bus interfaces, and twice the memory.

64k core memory in two 32k stacks (DE's last core memory product)
Clock frequency: 8 MHz (125ns clock period)
TTL integrated circuits, mainly 54LSxx parts family although some 54Sxx used in critical propagation paths
Microprogrammed processor with pipeline, and hardware-implemented branch prediction contained on four circuit cards: P1F - P4F.  Same "macro" instructions as the M362F-2.
Two -1553 bus interfaces
Same digital discrete and analog interface
300W power consumption (also "double" but not bragged on)

Comparing the FCC and EFCC SRU Layout

M362F-2 memory upgrade  The FCC's original 32k core memory quickly proved to be too small as more weapons capability was added to the F-16.  General Dynamics sought to increase the size of the memory, initially by requiring 128k of core memory to replace the two 16k core stacks.  This proved infeasible due to size and power constraints.  Instead, DE proposed a solid state memory backed up by lithium batteries.  The idea was sold to the (reluctant) Air Force as the only feasible way the memory could be expanded without major redesign of the FCC's chassis, power supply, and assorted other internal memory-related functions. The memory upgrade was implemented with 128k of CMOS static RAM (SRAM).  (USAF reluctance was centered on the lithium battery.) The redesign also included a hardware-implemented memory management function to accommodate a memory space larger than the native addressing range of the 16-bit processor.  Because the CMOS SRAM was faster than the original core memory the processor throughput increased slightly due to reduced memory access latency time. Power consumption was drastically reduced also.  (Core memory is a power hog.)  This expanded memory version of the computer was called the XFCC.

Delco Electronics lost the F-16 business as a result of losing the General Avionics Computer (GAC) competition to Teledyne Systems, Van Nuys, California.  This proved to be a wakeup call and spurred design activities resulting in the very successful M500 Mil-STD-1750B compliant processors implemented initially in twelve custom CMOS integrated circuits.  The M500 series eventually evolved to a single chip (in CMOS) -1750B microcontroller

In an unrelated note, the F-117 used three M362F-2 computers for assorted number crunching tasks.  These were the same computers used by the F-16 except they were painted a darker shade of gray and had a "Rose Monet" (pinkish) dot painted on the front for easy recognition.  About 1983, a few years following the end of F-117 production, these computers were replaced by a single, more capable number cruncher supplied by IBM Federal Systems Division.

Interesting, detailed info; is this personal recollection, or is there a source/sources that can be linked?

It's a personal recollection, Ralph. I was one of the FCC and EFCC designers at Delco Electronics in Goleta, CA.

Very cool - thanks for adding to the discussion!  And, welcome to Av.SE.

Those are some pretty specific details for "personal recollection". Wish I could manage to recall details like that! Welcome, indeed - now that you're revealed your background, much is expected of you. ;)

@FreeMan - Re: memory.  I have some Delco Electronics publications that help stimulate my memory.  Also, and probably more importantly, my participation on the FCC and EFCC project team at DE was at the very beginning of my career when I was right out of grad school, a time in life when people just soak up information.  And, I have been an aviation fan since my early teens, served in the USAF, and I would read the copies of Missiles and Space that my father would bring home from Lockheed (Burbank).  It all contributes.

Thank you, also, for your service! It's deeply appreciated.

F-16A/B had two computers (not counting the Firecontrol Radar).  It used dual Intel 8080 microprocessors for the Stores Management Set Central Interface Unit and Delco M362F (if memory serves me) (proprietary 16-bit) for the Firecontrol Computer, at least through Block 15B.  The 8080s were programmed in assembly language.  The FCC was programmed in JOVIAL J3B.  The Firecontrol Radar was purchased as a unit and GD had nothing to do with the software in it, as far as I know.
F-16C/D had four computers, again not counting the Firecontrol Radar, and used Zilog Z8002 and MIL-STD-1750A through at least Block 30.  All were programmed in Jovial J73.
F-16C/D later switched to Raytheon (actual Texas Instruments DSEG) Modular Mission Computers.  I do not know what processor(s) the MMC used.  They were originally programmed in Ada.  I do not know what they are currently using, but C++ would not surprise me.

What was historically unique about the F-16 was that it was the first use in a production aircraft of a purely Fly-By-Wire control system without a backup path of mechanical/hydraulic control linkage.
But to ask what CPU that Fly-By-Wire system used would be a mistake:  the original version was analog.
Only in the Block 40 series was the analog FBW system replaced by a digital one.
So yes, aircraft have for some time been a complex interaction of multiple electronic systems, and its likely there was digital computation for one purpose or another along for the flight from early on if not the start.  But this first production instance of electronics being at the center of the actual flight controls in between the pilot and control surfaces, was an analog system only later replaced by a digital one.

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What aerodynamic improvements allow variable geometry wings to be omitted on modern fighter aircraft?
This sounds like a duplicate of Why aren't variable swept wings being used in newer military aircraft? of which there is at least one existing duplicate, but this question is different.
This question is specifically about what are the aerodynamic improvements that allow the variable geometry wings to be omitted? This seems to be an interesting question, as the aerodynamic design of certain fourth generation fighters such as the Su-27 and F-15, appear to be relatively straightforward. It is certainly not clear to a layperson what enables this.
For this particular question, issues such as stealth, improved engines performance, etc. are not relevant.

Possible duplicate of Why do some military aircraft use variable-sweep wings?

It is more a tactical rather than an aerodynamic improvement.
Wing sweep is helpful when the top speed of the aircraft is well above Mach 2 and should be maintained for more than a few seconds. However, studies of past conflicts have shown that this speed capability was never used and almost never helpful even in hypothetical scenarios.
In addition, the lighter and stiffer structures made possible by carbon composite technology lower the maximum sustained temperature those structures can endure. Now the maximum Mach number is not only limited by aerodynamics, but also the stagnation point heating. In case of the F-22, the maximum sustained Mach number was lowered from 1.8 to 1.6 during development.

Further to what's been said above, there's also the weight and maintenance concerns. Having variable sweep wings adds a lot of weight to the primary structure, plus there is a lot of required maintenance required for the sweep machinery. Thus you would always try to avoid having variable sweep, unless it is absolutely necessary.
If we look at the two major US fighter aircraft with swing wings, they both had requirements which drove the design in different directions, leading to the need to adopt swing wings as a solution.
F111: The F111 needed to excel in the high speed, low level penetration role, but also be able to takeoff and land from short unprepared runways near the frontline. Short runways requires high-aspect ratio, unswept wings to lower the takeoff/landing speeds. At high speed, low level, the required lift coefficients are very low, as is the drag, due to the high velocity and air density. To reduce the drag, you need to reduce the frontal area and the wetted surface area, with very low aspect ratio wings. To meet both of these requirements the only way forward is to have variable sweep.
F14: Carrier take-off, and the fleet defense requirement, carrying the (very large) AIM-54 Phoenix a long way at high speed to defend carrier battle groups from Soviet Bombers with anti ship missiles. The carrier takeoff requirement is the same as the short runway, so you want high aspect ratio, low sweep angle. High speed means high M, so you want high sweep angle on the wings. As the AIM-54 is large, and needs a large radar and 2nd crewmember, the aircraft will also be large. Getting enough lift for takeoff/landing at low speed with a highly swept wing was not feasible, and therefore variable sweep wings were required.
More modern aircraft have generally not required the same set of capabilities, and we have a better understanding of the complexities and cost of swept wing aircraft. The aerodynamic understanding applied to the F14 and F15 was almost exactly the same, as they were designed at similar times, it's more that the F15's design mission ('not a pound for air to ground') meant that it was designed to be excellent in air to air combat rather than having short runway requirements or anything like that.

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What are accurate flight simulation models called? Is there a standard model?
I'm risking a question that might be "opinion based", but I hope it will not be seen as that.
This question is touched upon in the following questions:
What are the differences between PC and professional simulators?
Is this opinion on personal computer flight simulators too strict or is it actually true?
What are the limitations of the aerodynamics modelling used in flight simulators?
However, they fail to answer what I'm asking here, even if some particular things are discussed, such as stating that the coefficients are a mixture of CFD, wind tunnel data and flight test data, or that gust loads can be modeled with Markov matrices. However, no flight model package or system is named that I could see.
For racing simulators, or car simulators, simulators that are accurate are sometimes referred to as Milliken & Pacejka or "Pacejka-like", depending on the particular implementation. This appears to be a reference model that many simulators are compared against. I'm not a tyre simulator expert, so forgive me if I just said something that is stupid.
What about flight simulation? The flight model part of the simulator, is there a "gold standard", that is typically used with most respectable simulators?
Flight simulator games often talk about the "flight model" and describe it as accurate. While I can easily imagine that this flight model is based on some simplification of Navier-stokes,
I have software background myself, and although I didn't write a flight simulator myself yet, I understand the need to simplify, and that no simulation will ever be 100% accurate. All models will inherently have various trade-offs and make some sort of sacrifices. However, that doesn't mean there isn't a relatively accepted way of performing such simulation.
I'm not asking for an opinion based answer, "this model is the best". I'm certainly not asking for a list of "the best" flight simulators. I'm curious what underlying flight model is used -- that said, I wouldn't mind one or two examples of "this famous model described in the following academic paper, is used in flight simulator X and Y".
Basically, I'm simply curious if, in the industry, a typical model or small set (a couple) of models are considered either the actual "gold standard" or more of a de facto standard.
If there is no such thing, that pretty much every flight simulator does it differently, then I'll accept that as an answer. If there is a small set of terms that everyone in the industry is familiar with, I'd love to know then. It would let me easily search up papers that describe these systems.
I'll also accept an answer that lists an overview paper on the challenges of accurate flight simulation on modern computer systems.

14 CFR Part 60 regulates what constitutes a Flight Simulator Training Device (FSTD) for a particular aircraft model. FSTD is divided into two broad categories: Full Flight Simulators (FFS) and Flight Training Devices (FTD). A big difference between the two categories is FFS requires having motion cues, whereas motion cue is optional on FTD. Furthermore, the required accuracy of simulated flight dynamics (compared to flight test results) is much more stringent for FFS than is for FTD.
Each FFS and FTD is further divided into qualification levels (Level A to D for FFS, Level 4 to 6 for fixed-wing FTD). The higher the level, the more fidelity the simulator has to the actual aircraft. You may only get credit for Part 61 if you get training on the simulators with the appropriate levels for the systems/flight phases you are working with.
The simulator manufacturer must submit a Qualification Test Guide to the FAA to demonstrate that the simulator is good enough on aerodynamic model, flight deck, visual system, motion cue, sound system for the appropriate level.
Anyone can make a flight simulator, to any arbitrary level of fidelity; but making a flight simulator good enough for the FAA is hard and expensive. Read this for a more in-depth but lay-man explanation of the different qualification levels.

This is rather the formal side of things. Yes, it must be formally demonstrated that the simulator behaves according to the selected level. But it doesn't answer the core question: how such models are built.

Also, I disagree that the "single biggest difference" between FFS and FSTD is motion. In practice most FSTDs (at least of higher levels) have motion. Rather, it's a totally different level of certification effort and fidelity.

@Zeus The OP was asking "what is the gold standard for flight simulator", and not how simulations/models are made. I come from an airframer background so my expertise stops at the physical modeling. However, it's my understanding that Level D simulation models are supplied to high qual level FFS/FSDT, interchangeably. The optionality of motion cue is from cursory examination of  Part 60 texts, and seems to be upheld by the secondary source in the post.

'Supplied?' The model is the core of the simulator; all the hardware around it is secondary. It may happen that for companies that produce both FFS and FSTD (which is not very common) part of the model is shared, but often they are very different. In fact, most of FSTDs (except for the highest level perhaps) are built on 'gaming' engines such as X-Plane or P3D: for them, only 'generic' representation of a class of aircraft is required, and this is arguably a much more fundamental difference than motion.

@Zeus Upon cross-checking the Objective Tests between FFS and FDT of Part 60, I agree with your statement. Amended my answer.

As far as I can tell, there are two general approaches:

Solid body dynamic simulation with lots of experimental coefficients.
Most of the aerodynamic effects are simply summed for the whole aircraft, though the more advanced simulators need to calculate the wheel position and wing and fuselage flexing to get proper reactions mainly at landing. But it is still just a few elements, so it is not computationally intensive and can be very accurate if enough data is available.
The problem is getting that data. Lift curve for angle of attack and velocity, pitch moments for angle of attack and velocity, roll response to side-slip, pitch response to side-slip, lift response to slide-slip, pitch damping, roll damping, yaw damping, also ground effect and so on and on and on.
This seems to be the usual approach for the big simulators (as far as sources are publicly available; the vendors tend to keep the details confidential), because the manufacturer collects all the data during design anyway—from flight tests, wind tunnel and lately computational fluid dynamics situations.
It is also easy to show that it matches the behaviour of the real aircraft.
Disadvantage is that the data is often lower quality or not available at all for attitudes too far from the normal flight envelope, so the simulation is less realistic in upsets. But for flight training that does not actually matter, because the point is to avoid getting in those attitudes at all.
Blade element theory simulation.
In this approach, the geometrical model of the aircraft is broken down to elements and the aerodynamic forces on each are estimated using some basic lift and drag models and then summed for the whole aircraft.
This is not as accurate as the former if you have all the coefficients available, but allows still quite good simulation of aircraft that you don't have that data for, including aircraft not yet built, so it is sometimes used in design validation to get rough idea whether a proposed design will handle well.

But in general, the flight dynamics model is a tiny part of what a flight simulator must do. The instrument behaviour must be simulated with their lag and errors, effects of various system failures can be quite complex etc.
And then there is of course driving all the hardware of more realistic simulators.

The flight simulation model code for Level D sims is protected copyright from the manufacturer of the simulators.

What about flight simulation? The flight model part of the simulator, is there a "gold standard", that is typically used with most respectable simulators?

The gold standard is: accurate simulation of all of the physics involved in training a pilot on how the aeroplane and all of its systems respond. The complete code that complies with Level D is huge, about a 30 man year effort. A list of source code files would be:

Environment: atmosphere and navigational geography.
Aerodynamics: implementation of aerodynamic forces and moments in six degrees of freedom.
Flight dynamics: response of the aircraft upon forces and moments. Including aeroelasticity effects.
Ground reactions.
Crash conditions.
Aircraft systems interface (AFCS, AGPWS, FMCS etc.)
Mass and balance.
Aircraft systems. All the ATA chapters (autopilot, flight controls hydraulics etc etc). This is a huge amount of code, which also captures all of the system failure effect handling that pilots are trained in. Hydraulic system pressure loss causes reduction in max. surface deflection, depending on dynamic pressure etc etc.

I understand the need to simplify, and that no simulation will ever be 100% accurate. All models will inherently have various trade-offs and make some sort of sacrifices. However, that doesn't mean there isn't a relatively accepted way of performing such simulation.

The accuracy, trade-offs and sacrifices are determined by: what standard is the software written for. Compliance to the standard must be demonstrated.

If there is no such thing, that pretty much every flight simulator does it differently, then I'll accept that as an answer.

The understanding and capturing of the underlying physics is the only correct way of writing simulation code. Used to be a bit of a black art in the nineties, tools like Simulink make constructing block diagrams and system responses a lot more accessible. But every new simulator company must re-write from the ground up, using physics simulation.

The last paragraph is not quite true. Even physics simulation can be done on very different levels; Jan's answer mentions two of the most common approaches. But in the end, any 'cheating' is acceptable if the model behaves like real, and it can be demonstrated. Most rigid interactions are 'cheated' rather than solved physically. The OP mentions Pacejka's approach: this is outright cheating (this is a 'magic formula' that works in practice, instead of solving the tyre physics), and it's very common. As for rewriting, like with most software, there are freeware simulation engines out there.

With the compliance, it may be confusing to say 'standard'. Rather, validity of the behaviour must be demonstrated (to the extent stipulated by the simulator level). This may not be trivial because validity of the source data must also be demonstrated. But the software writing standards per se for simulators are not specifically regulated, unlike the avionics and other airborne stuff.

@Zeus "As for rewriting, like with most software, there are freeware simulation engines out there" I know, I've used one for testing the aero and flight dynamics model of a nav trainer helicopter, including systems simulation, that we wrote and is used for training at the moment. Not on a Level D simulator though, that is a different kettle of fish and you don't get away with much cheating at all without introducing bugs somewhere else that will get you into trouble

Well, of course, one rarely just uses someone's model, even if it's allowed. But like with any established software (like OS), it's rarely happens that a new project 'must rewrite from the ground up': there is enough stuff to build upon, even if by just using the experience and approaches.

As for cheating, I can assure there's plenty of it even at Level D. There are things that are simply not realistic (or not worth) to calculate 'physically' in real time, and 'cheating' can give better (more accurate) result than 'honest' simulation. For example, when the model assumes wing flex at certain frequency, this is, by and large, 'cheating': it's not realistic to do FEA of the airframe in real time. Similarly, it may be more reasonable to induce skidding directlty instead of simulating rubber tyre properties. Failures and other random events are also in this category.

@Zeus The company I have experience with had rigid guidelines for modelling and sailed through all Level D requirements, EASA + FAA + all others. Data from OEM, with additional data from dynamic wind tunnel testing at very large angles of attack and sideslip. Full stall recovery and UPRT implemented 10 yers ago. Also the way we practiced physics simulation of the flight controls, each aircraft was analysed and the particular hardware configuration was implemented. With degraded systems functionality etc. Sure the industry at large tries to get away with minimum effort.


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Is it practical to fly IFR into Hawthorne Airport (KHHR) in IMC?
I am an IFR rated pilot flying 172s and 182s out of Hawthorne Municipal Airport (KHHR) in Los Angeles. I've heard from other pilots that in actual IFR conditions (IMC) it can be difficult to get sequenced for an approach due to the proximity of inbound jet traffic at nearby LAX, perhaps due to separation requirements for IFR aircraft combined with the parallel approaches or the two airports:

What really happens when both airports are landing to the west as is most often the case? My worry is that if I'm returning home on a cloudy day, I might be forced to land at Torrance airport (KTOA) and then Uber it back to my car at Hawthorne.
I've heard from other pilots that they have returned to the area and been told they will have to hold for hours, which obviously makes the Hawthorne LOC approach useless in a practical sense.
Ideally, this question would be best answered by someone who works as an approach controller in the LA basin.

If you're an AOPA member, check out the recent discussion forum thread about doing the SoCal TRACON tour. It sounded very good, and a great way to get to put your questions to ATC directly.

Without pulling up the SOP/LoAs between HHR and LAX and SoCal Tracon, or working at those facilities to see if there's a better option, and just looking at the positions of the airports and their approaches, I'd say for an actual approach into HHR, you'd have to be vectored into the LAX arrival stream and sequenced like that.
From the approach plates, at the HHR Final approach fix you might have enough separation from LAX's 25L/R, vertically, but once you get to the end of the runway there isn't enough separation between the procedures.
Now if HHR tower can see the LAX traffic and apply visual separation that can change things as well, such that they are making sure you don't get too close, to the other final, as long as wake turbulence isn't involved.
Beyond the theoretical, just call the tower or SoCal Tracon and ask, or schedule a visit and ask.


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How does a ring paper airplane fly for a long distance?
My friend showed me something which he called the glider. At a first look, it looks like it will crash, but it flew quite well:

I was surprised to see it go about 25 meters (82 feet), as it was nothing but a big ring made out of paper. I thought of how it could fly, but I had no idea. I knew that it could go through air with least resistance, but it wasn't supposed to fly.
Can someone explain me the logic of how this flies?

Was the paper heavier at the front, either by folding or by adding a paperclip?

@PeterKämpf, nope, there was nothing. It was the same throughout.

@mins. Mm, actually I am an absolute beginner. I don't know the difference. Basically my friend just held the ring in his hand, and threw it. He held the curved paper part in his hand. Also, it is not launched in rotation.

@mins, no the youtube video, this is different. The one I have posted in my question uses an A4 size paper cut into half (by length) and the edged joined by glue. Also, I think the throw was axial.

I saw something similar to this before as one of my friends rolled a piece of paper up into a cone and it "flew" well too.

The edge of the ring facing the viewer on your picture is opposite to flight direction, right? If you look closely, you can see that the far edge has been folded onto itself to add some weight at the front. Dare your friend to throw it holding it "backwards": If it would be the same throughout, it would still fly. I bet it won't.

@Peter Kämpf It looks to me like the same thing has been done at both ends of the cylinder.

@Peter Kämpf  I'm wrong about that. There's a video on how to make it.  Several folds in front. Half of the weight of the paper is in the front edge.

Demonstrated on TV show QI (2:00 onwards)

I play with these and in order to fly you need to add a little rotation. The rotation adds a gyroscopic effect and prevents tumbling in the same way that the  rifling in a gun barrel adds rotation to a bullet to stop it from tumbling. Experiment for yourself and try making one (with a leading edge fold for weight) and try it with and without spin.

No takers? Then I'll bite.
This question addresses how paper airplanes generate lift. This lift is caused by higher pressure on the lower side and lower pressure (suction) on the upper side, and if both are added up, the chordwise pressure distribution is shown below:

Flat plate pressure distribution (picture source)
If you collect all the local forces in one point, the lift acts at a quarter of the wing's chord. That is why the forward half of the paper which in the end will be bent into the ring needs to be folded onto itself: The center of gravity of the wing is also at one quarter of chord. Thus, the lift and the weight will act a the same station and no pitching moment results.
To achieve a flight distance of 25 m, your friend threw the ring with some force to give it a high initial speed. This speed would allow it to create enough lift with very little angle of attack, so it did not suffer from flow separation initially. This resulted in low drag and the high launch speed gave the ring some kinetic energy.
Friction slowed the ring down gradually, but the decrease in lift at lower speed was compensated by a gradual increase of the ring's angle of attack. Why would the angle be just right to prevent the ring both from rising and dropping, you might ask? Any imbalance between lift and weight would add a vertical acceleration which would immediately change the angle at which the ring hits the flow. Its chord length and inertia would prevent it from rotating nose up or down, so it would pretty much stay on its initial path until most of the speed was eaten up and the angle of attack was so high that separation sets in. At this point, the drag would go up and speed up the deceleration process, and the ring would sink to the ground.


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What are these four little things between the horizontal and vertical stabilizer on the B737?
I was looking at some photos today and came across this one:

Image source
I noticed four little things in between the horizontal and vertical stabilizer. Could somebody explain me what those are?

From what I see, they are not antennae. I might guess that they are static-relievers but I may be wrong.

We also have this question which is very similar http://aviation.stackexchange.com/questions/24427/what-is-this-device-protruding-from-the-rear-of-this-787

do you mean the metal plates over the fuselage?

Personally, looking at that picture I would asked about tube like formation on the back.

What are those "tube-like" structures ahead of the vortex generators ?

@JorgeAldo I would say that they are side-looking radar antennas. (this aircraft is part of the indonesian air force, probably a patroller/airborne radar station).

The objects are vortex generators. In general, vortex generators serve to introduce turbulence which helps to prevent flow separation. More information here:

This was to reduce elevator and elevator tab vibration during flight to increase their hinge bearing service life.

As mentioned on the site, you can see these on any 737 version from the original -100/-200 up through the NG. Better image from same site:


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How is close formation flying possible with things like thermals and turbulence?
This weekend the Blue Angels came to town. The weather was less than perfect. A quick look at SkyVector during their show indicated there was an active AIRMET for moderate turbulence.
This made me think about the effects of weather (namely phenomena that can change affect the physical location of an aircraft in flight like turbulence and thermals) on planes flying in extremely close formation.

How is close formation flying possible with conditions like this?
Is the net effect of this type of weather minimized for planes of this type?

They are close enough that a thermal that affects one should affect the other the same way.

@RonBeyer There's a limit, but that's my regreet: I don't have the Sop anymore.

Disclaimer: I'm not a Blue Angels pilot so don't know their exact procedures/limitations, but have done a fair share of military formation flying.
Answer:
It is possible, it's just a more "bumpy" ride. Not a huge issue if the turbulence is not severe enough (or of a "rotor" type) to cause uncontrollable rolls in three dimensions.
When flying formation you are separated from other aircraft in three dimensions. In turbulent weather conditions the aircraft position usually results in vertical displacement, not so much in horizontal displacement.
This means the aircraft will not roll into the adjacent aircraft (the roll effect happens when you fly into wake turbulence or through rotors for example), but do more of an "up and down" jump.
The same air disturbance will effect several (if not all) aircraft in the formation, sometimes with just a tiny delay.
Also, the heavier and faster the aircraft is, the less effect the same amount of turbulence will have. If weather doesn't permit a small single-engine piston to fly formation, it may still be workable for jets.
Ultimately, it is up to the flight lead to:
- determine if conditions permit close formation
- fly the formation to avoid known hazards (for example if the wind is from the N, don't fly on the S side of hills/mountains)

Would they simply add more spacing if the air is turbulent, or have they drilled so much on a specific spacing that adjusting that would throw everyone off? (best guesses, as you're not a former BA pilot, would be sufficient)

You could do that, but I don't think a display group would. When flying close formation you keep your position relative to other aircraft based on visual cues. I don't know specifics for F-18s, but things like: keep wing tip just below the canopy, see only the bottom half of horizontal stabilizer, keep tail light aligned with rudder etc... BA (and other display teams) practice so much that keeping position, even in turbulence, becomes second nature.


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Why does the Piper Cherokee (PA-28-140) engine have such low horsepower despite the very large displacement?
I'm sure this question could apply to many other plane engines, but specifically I'm looking at the Piper Cherokee PA-28-140.
Surprisingly, it appears that the engine used, the Lycoming O-320, has ~320 cubic inches of displacement but only puts out 150hp.
When comparing the horsepower per cubic inch of displacement (CID), the ratio is quite low. We have the technology (and already used it in vehicles in the 50's, 60's etc.) to achieve a better efficiency than that.
Why is this engine (and perhaps other models) so inefficient?

There seems to be a general pattern I've noticed that small GA aircraft engines tend to produce 1 hp per 2 cubic inches of displacement.  This isn't too far off that typical pattern.

Remember that the IO-320 is a 4 cylinder, 5.24L engine, that's a huge 4 cylinder, with a compression ratio (max for that engine series) of 9:1 (or as low as 7:1). Auto engines use compression ratios between 10:1 and 15:1 (or higher for diesels).

Because the engine speed is limited. Car engines easily have twice the RPM of an aviation piston engine, so they can produce twice the horsepower from the same displacement. Add turbocharging and the gap widens even more.

One reason is not so much can you build a reciprocating engine with higher horsepower or better efficiency, but can it do so with a very high reliability for extended periods.  Most automobile engines are run on average at 20% power with very brief higher power outputs whereas aircraft engines are routinely operated at 65-85% power and are expected to have a mean time between overhauls of around 2000 hours at these power settings.  To accomplish this, most aviation engines operate at slower speeds and much greater displacements than traditional automobile engines do.  Put simply, manufacturers, just put a huge engine on these airplanes that drones, along at a power setting far below its ultimate limit for a high degree of reliability.

Aircraft engines operate at those high power levels because they produce so little power, no? If you double or triple the power it put out per displacement (so that it matched car engine levels) then they would only need to operate at 20% power.

What happens to the unused 80% in a car? Same thing here. What do you mean about it not things not being free? Either you have an efficient engine and you only use a fraction of its power. Or you have an inefficient engine that runs at max power. Torque is generally lowest at low RPM. Highest around the middle of the power band, then tapers off. HP is mostly linear, but drops off quickly after peak. Look at the power curves for some engines http://services.edmunds-media.com/image-service/media-ed/ximm/?quality=85&image=/ford/shelby-gt500/2013/fe/2013_ford_shelby-gt500_r34_fe_1011126_600.jpg.

You're looking at this problem all wrong; its not that an automobile engine 'wastes' the other 80% available power; it just doesn't need to operate at those higher power settings.  So a car with a 240 hp engine in it only needs to use 40-60hp during typical operation on public roads at sanctioned highway speeds.  And though a typical automobile engine redlines around 6000-7000 rpm, it's usually operating at 2000-3000 rpm while driving.  The faster the engine operates at, the more stress it's parts are subjected to and the shorter the service life is.

it's also far more critical that an aircraft engine be more reliable than an automobile engine as the consequences of an engine failure are more dire for an aircraft than an automobile.

A modern aircraft propeller must be designed so that the tip speed is less than supersonic so as to avoid issues with noise and performance. Most small plane propellers are about 6 feet in diameter and are thus limited to about 2700 RPM. The engine designer may thus connect the crankshaft directly to the propeller and limit engine speed to about 2700 rpm, or use an intermediate gearbox to allow higher crankshaft speeds.
Because the propeller has very high rotational inertia and wants to turn at a constant speed while the engine supplies power in a few pulses during each revolution, the wear and tear on a gearbox may be quite extreme. As a result, a high-speed engine plus robust gearbox is often not worth the extra complexity as compared to a simpler direct drive solution. (see "Continental Tiara" and "Porsche PFM 3200")
Of course, as engine power is proportional to the product of the torque and speed of rotation, the slower turning engine produces less power for a given displacement. But experience has shown that slower-turning direct drive units are often as light and more durable for a given power output when compared to geared units.

For reference: a 2-foot diameter car wheel at 60mph/~100km/h is about 840rpm.  At 100mph, 1400rpm.  (According to google's hit on a quora question; I didn't check the math but those sound reasonable).  So the car's inertia has less mechanical advantage on the drivetrain than a prop has on its drivetrain.  (I assume there are other huge differences in drivetrain that I don't know about.  Car engine flywheels help with this some, right?).

@PeterCordes (840 / minute) * pi * 2 feet in mph — yep, that works. And 24" is a reasonable enough tire diameter.

I think this argument makes much more sense than ymb1's: the specific output is low because the RPM is low (absolutely speaking), not because it is high relative to the engine's rated maximum.

Well the truth is plenty of aircraft do use high speed engines which are geared down to drive a propeller.  Take for example a PWAC PT-6 Turboprop engine:  The power turbine typically operates at 4500 RPM and drives a reduction gearbox with an output to the prop at 2000-2200 RPM.  PT-6s have logged more than 4 million flight hours and are very reliable.  The Rotax 912 is another high speed reciprocating engine, popular with ultralights and LSAs, which uses a reduction gearbox for it's shaft output.  Gearbox reliability is not a problem if design correctly.

You can certainly design gear boxes to be reliable (especially when they provide a single, fixed reduction ratio). I'd note, however, that 4500 RPM isn't particularly fast by automotive standards. It's faster than a typical car engine runs most of the time, but red line on a reasonable high performance car engine is typically closer to seven or eight thousand RPM (and F1 cars can hit around 12.000 or so).

@CarloFelicione I think Tom's point was that the unevenness of a piston engine's torque (combined with the rotational inertia of a propeller) poses a problem for the gearbox. That's obviously not an issue for a turboprop. Though I agree that it also shouldn't be too much of a problem with a modern piston engine, especially for one that runs on more cylinders with higher RPM.

RPM, RPM, RPM.  If you want 1,000 HP out of 1.5L, you run it as 10,000 rpm.  An earlier poster referenced prop tip speeds being a limiting factor - true.  However, there are geared aircraft engines which permit the engine to run at higher RPM without overspeeding the prop.  These are the same displacement as their non-geared brethren but make more horsepower for the simple reason that they are burning more fuel/air and making more heat.  They are also more expensive to maintain and are seen on larger twins where they remain cheaper than adding another engine.
Not related directly to horsepower, but the design of A/C engines have a greater bore to stroke ratio then auto engines - typically 4:3 while auto engines are closer to square or with even longer stroke.

You'll rarely see a stroke longer than the bore on a modern automotive engine, and 4:3 isn't particularly rare/out of line either. Even truck engines can be in the same range (e.g., GMC used to have a 430 CID engine with a 3.47" stroke and a 4.44" bore).

This is a rather old question, but I think that there are two important reasons not fully answered.
Firstly: the volume of the engine is in itself not relevant. The total of other factors such as the size and weight of the engine, the power output, reliability and the fuel effiency are what matters.
You could use smaller volume engines, in fact some are available. They run at a higher RPM and "gear" down to the propeller at a max around 2400 RPM (the tips of the propeller should stay below speed of sound). This might lead to a better total performance, say lower weight or better fuel economy, but may be offset by the higher complexity. As always, engineering is about compromises between different requirements.
Secondly: aviation engines is a very small market with a lot of regulations. If you compare to say cars, I would guess that total number of aviation engines sold in a year is less than one of the larger car manufacturers makes before lunch on a normal day. Add in that it takes a lot of paperwork to get the engine approved as replacements in older airplanes. This means that any new engine design faces a hefty upfront investment in order to gain a quite small payback in the short term, not a very good sell to capital investors.


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What is the difference between Cruise and Service ceilings?
I am looking at aircraft performance data. There are charts for "Cruise Ceiling" and "Service Ceiling."  What is the difference?  (The cruise altitude is lower.)

Ceiling refers to the density altitude the aircraft is capable of reaching under a set conditions (configutaion etc.). The main difference between service and cruise ceilings is the aircraft rate of climb at those altitudes. At service ceiling, the rate of climb is 100 fpm. From FAA pilot handbook:

The service ceiling is the altitude at which the aircraft is unable to climb at a rate greater than 100 feet per minute (fpm).

The cruise ceiling, on the other hand, is the altitude at which the maximum climb rate is 300 fpm, though I've not seen it used much. The absolute ceiling is the maximum altitude where the aircraft can sustain level flight. From the same document:

At the absolute ceiling, there is no excess of power and only one speed will allow steady, level flight.

The image below shows the data for A-10.

Image from forums.eagle.ru
Note that these values depend on the aircraft configuration.

Do you have a source for your cruise ceiling definition? I have seen references that do support your definition, but Peter's is different.

@JonathanWalters It's in A7 Pilot operating handbook, for example.

Service ceiling is generally understood as the maximum altitude at which the highest vertical speed drops below 0.5 m/s or 100 ft/min. This assumes a clean aircraft flown at the best rate of climb airspeed.
Cruise ceiling is the maximum altitude at cruise speed. Cruise speed is one of the design speeds as defined by FAR Part 25, §25.335 and is determined by the manufacturer. Also, the remaining climb speed at cruise ceiling might differ and is defined by the manufacturer if a cruise ceiling is given at all.
Since the cruise speed of an aircraft is generally higher than the best rate of climb speed, the cruise ceiling is below the service ceiling of that aircraft.

Do you have a source for your cruise ceiling definition? Aeroalias' answer gives a different definition, and I am not aware of any sources that support your definition.


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What does RPM mean in a propellor aircraft, compared to a jet aircraft?
What is the practical significance of speed RPM in a propeller aircraft for a pilot? I am not able to correlate it with RPM in jet engines?

I don't really understand your question. By "propellor aircraft" do you mean "piston engine"? Or turboprop? How and why are you trying to compare RPM values? Can you give a specific example to help clarify your question?

@Adder. If you are trying to equate power in a prop aircraft, with thrust in a jet powered aircraft, just knowing the RPM of the prop, and the RPM of the gas turbine, is not sufficient information. So, the answer is, there is no correlation between just these two parameters.

There are three primary cases for use of RPM in aircraft that differ by engine type: piston, turboprop and turbofan. In all three cases RPM refers mainly to two speeds, propulsor and engine. They may be indicated in percent of maximum (usual for turbine) rather than as a specific speed (usual for piston).

Piston engine- the propulsor is the prop and the engine speed measured is the crankshaft. Usually there is no gearing and both are the same, so there will only be one gauge in an aircraft. Notable exceptions are warbirds with high power and correspondingly large props.

Turboprop - the propulsor is again the prop and the engine speed being measured is the turbine output shaft, fixed to the low pressure compressor. This is input to a gearbox since turbine speeds are too high to be useful in directly driving a prop. There can be two speed gauges in the aircraft.

Turbofan - the propulsor is now the front fan and the engine speed being measured is usually the core, the high pressure compressor. The front fan historically runs on the low pressure compressor shaft. PW inserts a gearbox between the front fan and the output shaft, making the drivetrain resemble the turboprop configuration. More complex engines may have three shafts, all running at different speeds.

RPM in a propeller aircraft is typically under 2400, as the prop spins at the engine RPM speed. Above 2700 the tips of the propeller begin going supersonic and make more noise than anything.
Jets engine spin much faster, as discussed here
Which are typical rpm values for aeronautical turbines?

Just two nitpicks: 1) the prop spins at a multiple of the engine speed, as a lot of them have a gearbox in between. 2) The blade tips will go supersonic at lower rpm if they are longer, so giving just an rpm number as a limit is misleading.

What @AEhere said (and given the answer's focus, I'm not sure I'd call that just a nitpick). The specific value will depend on gearing and propeller size. As one example, for the plane I'm flying, the magneto check is done with the engine at 4000 rpm, warmup is 2000 rpm, and idle is somewhere around 1800 rpm. (I don't recall off hand what the cruise rpm is.) And of course, while it'll be a multiple of the engine rpm, it won't necessarily be an integer multiple (and technically it doesn't need to be a multiple >1 either, but I'm not sure what would be the value of a sub-1 multiplier).

in propeller aircraft, the practical significance of propeller RPM is it is one component of the equation that determines propulsive power, the other being shaft torque.
If the propeller has a fixed (nonadjustable) pitch, then the RPM of the propeller is a simple indication of the power output of the engine, and the pilot manages engine power by adjusting tachometer RPM with the throttle.
Controlling the power output of a turbine engine is a more complicated process, and depending on the type of engine and its application, the pilot may use pressure measurements in the different stages of the engine or ratios of those pressures or shaft RPM's expressed as percentages of full power to manage the power output of the engine.


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What is the difference between a "compass swing" and "check swing" procedure?
What is the difference between a "compass swing" and a "check swing" procedure?

Based upon the terminology I have heard over the years, a compass swing is the alignment of the compass, normally using a compass rose painted on a surface. The compass is adjusted and a deviation card is created for cardinal points on the compass.
A check swing, as I have heard it referred to, is the checking of the compass, and done with the aircraft running, and electrical loads on, similar to a compass swing, but without the adjustment.  The deviation card is verified, but there is no intent of adjusting anything.  This was typically done before we ferried aircraft over the ocean, where there would be extended dead-reckoning (before GPS and LORAN), and where flights were close to the polar regions, and compass operation needed to be verified prior to the flight.
Not that a compass is worth much near the poles (grin).
My guess is that check swing is merely a slang term, whereas compass swing is well understood and extensively documented.

In the US maintenance by pilots is governed by FAR Part 43 Appendix A, Paragraph (c). Compass swings are not included in the list of preventive maintenance pilots can do, so must be done by a certified A&P. Compass checks involve no maintenance, so are perfectly fine.

@JScarry, you are correct.  I did a double check.  I also found that when I swung the compass the mechanic put an entry in the log, along with a copy of the deviation card I created.  So I guess that is mechanic supervised maintenance.  Thanks for pointing out.   I removed my addendum, which had that error.

I happened to notice a bunch of logbook entries with the deviations in the books I was reviewing over the weekend. It seems they often did a compass swing after installing new avionics, which makes sense since the magnetic fields generated by the avionics might change.

If a HF radio was added, like for oceanic crossing, we would strap the radio in place, and swing the compass.  two degrees off makes a big difference crossing the pond.  Topping things off, most of the planes did not have working AP.  So it was about 28 hours of stick time for some of them.

A compass swing involves adjustment by a qualified person. A check swing can be done by anyone to verify the serviceability of the compass without adjustment. Check swing can be done anytime the compass is suspect or, if required for, annual check. if no adjustment is required no certification by a compass qualified person is required.


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How many of this aircraft were manufactured?
What was the total quantity issued of this model aircraft (including any modifications).

From the registration ID, this is a Convair CV-340, replacement of the DC3. You may look at the CV-240 family (240, 340, 440) on Wikipedia. 312 units of the CV-340 were build.

You can also work out the model by noting that "The flying dutchman" indicates airline KLM (also marked on the fuselage) and looking at "Fleet History" in the Wikipedia page for KLM. By clicking the links for the various aircraft operated by KLM you'll eventually find a matching aircraft in the "Convair 340" and "Convair 240" operated by KLM from 1953-1964 and 1948-1959 respectively.

@RedGrittyBrick That's how I found it out. On an unrelated note, I don't get why KLM would name its aircraft after a ghost ship.

@aeroalias, It's not associated with the aircraft, but with the airlines (sort of slogan), Now the title of the airline's flyer and loyalty program. I like this one.

@aeroalias: The PH-TGI was actually named the "Vincent van Gogh" after the painter. Source. The same plane  in different colors, may 1962. The PH-TGI is a Convair 340, according to the Nieuwe Leidsche Courant from September 9, 1953.

The aircraft looks like the Convair CV 440 Metropolitan (maybe CV 340) a variant of the CV 240, in operation with the KLM. According to Wikipedia, a total of 1181 of all the variants were built.

Apparently is was built as a -340, and in 1955 upgraded to CV-440 specs when it was renamed to the PH-CGI. Seems fair to treat that as a modification.

@MSalters Thanks for the info. I wasn't sure of that. Thanks for clearing it up.


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How can I correctly decode CPR positions from ADS-B frames?
I am decoding Compact Position Reporting (CPR) positions from ADS-B frames (the airborne position and surface position frames).
Most of my results give realistic longitude/latitude values, but some return things like 0,781518968485169 177,220642089844; or 1,76377118644068    -0,664306640625; or 2,12129237288136 -2,838134765625.
I have a feeling like I must be missing some kind of validation or some kind of frame type identification. Where can I find more information about the decoding process?

I think some people are unfamiliar with ADS-B and didn't realize that your question had to do with transponder signal coding.

Welcome to Aviation.Se!

Do you use global decoding or local decoding?

Can you explain how you do the decoding, it is impossible to see from result what is wrong.

Are you using a specific library like Dump1090 - we are having similar issues lately

No, we were using homemade C# code to decode the frames.

We were decoding Airborne Position frames having TypeCode 0, 9-18 and 20-22.
Apparently, frames with TypeCode 20-22 and possibly 0 have to be decode a different way.
(For now I think that I get enough information out of TypeCode 9-18 frames so I'll just skip the other ones.)

Type codes 20-22 have the same coding as 9-18, all are Airborne Position messages. 19 is Velocity, there is no CPR in there. 0 is a "no position message", there is basically only altitude in there. 1-4 is identity & emitter type. 5-8 is surface position & velocity. 5-8 have a slightly different position coding than 9-18,20-22.

In addition, if you see type 20-22 I'd expect there is something wrong with your receiver or decoder as these messages are really, really rare.

Sorry I mistyped '8-19' for '9-18'. We don't receive 20-22 now, but why would that mean there is something wrong.

20-22 has very high position integrity which is not achieved with current day avionics.


text:
When is a transponder required below class B airspace?
I just wanted to clear up questions I have about flying within the 30 mile veil of Class B. My understanding is that the Class B is the inverted wedding cake, having tiers as the airspace moves towards the center. If I remain outside of the published altitudes, yet within the 30 mile veil, am I required to have a Mode C transponder?
What are the requirements if I have no electrical system on my plane?

Short answer: mode C is required unless the aircraft was originally certificated without an electrical system; but ATC can always authorize you to operate within the veil anyway.
Your question isn't exactly about class B airspace itself, it's about the mode C veil; those are separate things although they do go together. (Note that from 1 Jan 2020, you'll require ADS-B out to operate within the veil so the name will be a bit outdated).
The AIM section 3-2-3 has a good explanation of the transponder requirements but the regulations are more precise and complete. They're in 14 CFR 91.215(b)(2) (and see 91.225(d)(2) for the ADS-B ones) and they apply within the veil, not within class B:

All aircraft. In all airspace within 30 nautical miles of an airport
  listed in appendix D, section 1 of this part from the surface upward
  to 10,000 feet MSL;

That means that the shape of the class B airspace has nothing to do with the shape of the veil, which is always a cylinder 30nm across and 10,000ft high.
As for operating without an electrical system, that's covered in the same section. If the aircraft has no electrical system by design then you can operate within the veil but clear of class B:

any aircraft which was not originally certificated with an
  engine-driven electrical system or which has not subsequently been
  certified with such a system installed, balloon or glider may conduct
  operations in the airspace within 30 nautical miles of an airport
  listed in appendix D, section 1 of this part provided such operations
  are conducted—
(i) Outside any Class A, Class B, or Class C airspace area; and
(ii) Below the altitude of the ceiling of a Class B or Class C
  airspace area designated for an airport or 10,000 feet MSL, whichever
  is lower; and

If the aircraft has an electrical system but it isn't working (i.e. you're taking the aircraft for maintenance) then you can request authorization from ATC to operate within the veil:

(d) ATC authorized deviations. Requests for ATC authorized deviations
  must be made to the ATC facility having jurisdiction over the
  concerned airspace within the time periods specified as follows:
[...]
(2) For operation of an aircraft with an inoperative transponder to
  the airport of ultimate destination, including any intermediate stops,
  or to proceed to a place where suitable repairs can be made or both,
  the request may be made at any time.

You can also request ATC authorization if you don't have a transponder, or if it doesn't have mode C. But if you don't have a transponder, you need to request authorization at least one hour in advance, you can't request it "at any time" as you can in other cases (see 91.215(d)(3)).

From AOPA

For flying into a Mode C veil without an operable transponder, the pilot needs to telephone the appropriate radar facility for the Class B airspace and ask for permission to make the flight. Upon agreeing to conditions (including direction of flight and altitude), the pilot will be given a code number that he will mention to the controller upon initial radio contact.

So you will need a radio but that does not mean you need an electrical system, a hand held should suffice.


text:
What is the physical definition of turbulence?
To aviators, the definition of turbulence by measuring the results is widely known: example. But if we were to generate a turbulence forecast as a meteorologist, there need to be some kind of physical parameters that constitute the different levels of turbulence.
Maybe something like "wind vertical velocity" or "vorticity" etc. ...
What are these parameters and what different levels are defined?

Image source
Turbulence is merely the measure of Reynolds number of a fluid at rest or motion ( (turbulence occurs when fluid is in motion) let's say you light up a cigarette and when the smoke is at the tip of cigarette you see it flows upwards uniformly and this is because the fluid is in laminar flow and as it expands into its vicinity it becomes turbulent. so what is the physics here? basically, reynolds number defines if a fluid is laminar or turbulent. Reynolds number is defined as the ratio of inertial to viscous forces experienced on a body. when divided it gives a number without any units. If the Reynolds number is less than 2100 then the flow is laminar and greater than 4000 it's turbulent and if it's between 2100 and 4000 its transitional flow (which is rare and instantaneous)

Thanks, and which number would you map to the different levels of "moderate" and "severe" ?

The numbers you mention are for pipe flows, and it would be good to quote your sources (both for the image and the numbers/text)

actually, I'm new to stack exchange and I learned about different flows in computational fluid dynamics I got this picture from link

OK, welcome to Aviation SE, and thanks for the input :) I've included your source in the answer. That being said, this question mainly focuses on weather related turbulence, of course this is the same phenomenon as in the pipe flow, but it might not be the answer the OP is looking for.

oh alright, I'll try to stick to the details of the subject next time. thank you :)

It may also be worthwhile to point out that the Reynolds number requires a flow speed and a characteristic size for its computation, so the Reynolds associated with a large aircraft may be in the 1e7~1e8 range, while that same aircraft will have a much lower Reynolds on its boundary layer or near the wingtips.

@AEheresupportsMonica That is not really true. The characteristic speed for weather generated turbulence is some measure of the characteristic velocity differences. Near the ground it will be the wind speed.

authors: Vladimir F Героям слава


text:
When replying to ATC, callsign first or last?
When I was in flight school, some instructor told me that it is the unknown ICAO standard that you should reply to an ATC call with your callsign first, like this:
"XYZ123, climb FL350"
- "XYZ123, climb FL350"

But I only know one airline that operates like this. In all other parts of the world, I hear:
"XYZ123, climb FL350"
- "Climb FL350, XYZ123"

So can someone tell me the definite ICAO standard on that and where it is written down?

Standard aside, I think it is important to do what everyone else is doing. It makes your response align with what controllers and other pilots expect.

I have no idea what ICAO says, but the FAA says "either at the beginning or at the end of your transmission" (AIM 4-2-3(c)).

@Pondlife, do you have a link?

Sure: https://www.faa.gov/air_traffic/publications/#manuals

Does this answer your question? Is it bad practice to say my callsign first in a transmission to ATC?

The standard procedure for a pilot:
1. Who do I want to talk to?
2. Who am I?
3. Where am I? (not necessarily needed after initial contact)
4. What do I want?

After initial contact, some of the requirements may be ignored when back and forth communication is happening, without other airplanes talking to that ATC.
For read back, it is needed that the pilot repeats the instruction to clarify. The call sign can be at the end for confirmation.
An example of that is here:

Controller: Callsign, climb (descend) and maintain new altitude
Pilot: Climb (descend) and maintain new altitude, callsign


text:
Who operates a 707 with Boeing factory livery?
I have just seen a 707 depart from HTDA Dar es Salaam that has the tail painted with the Boeing colors. Any idea of who the plane belongs to?
Livery like this one:

Obviously 707 instead of 777 painted on it.

Official airplane of Democratic Republic of Congo
Today, the Democratic Republic of Congo uses a B707-138B, registered 9Q-CLK: LK for Laurent Kabila.
Laurent Kabila has been Congo's President until his assassination (he was succeeded by his son).
This is an airplane with a long history (and some controversy). She has now 707 painted on the tail, but not the Boeing markings. Thanks to our friends at Airliners.net several pictures are available:

(source: Airliners.net, Mark. Szemberski)
Once this a/c had no logo at all, here in 2008:

(source: Airliners.net, Weimeng)
On Youtube too: B707 9Q-CLK .
Qantas in the past
Back in 1959 when it entered service at Qantas as VH-EBG, the first B707 with a 5th engine in a pod-pak:

(source: Qantas Heritage Collection)
More on the curious engine exhaust and the pod-pak.

Side question, but what the heck is up with that engine on left of the last picture? It looks totally different.

@DavidGrinberg The trick is that on the right it's no engine at all. It's a Pod-Pak, an encasement for spare engine carried just in case ground crew needs one.

@Agent_L: The same pod-pak with the crew. Another picture of the pod-pak. And this overall view showing the location. Hey this was 1959! Still flying after 56 years, albeit without the spare engine.

But what's up with the engine on the left, why so many exhaust holes?

@Alexus: This is explained by DeltaLima in the linked question: It's for noise reduction.

@Alexus, it's not an engine, it's a missile pod ;-)

What I don't get is why they still fly the boeing 707

@Ethan Because the DRC is a fairly poor country and doesn't have enough money to fly its president around in something more modern? It has an economy slightly smaller than Idaho's -- can you imagine the Governor of Idaho flying around in, say, a 737-800 series?

"Quantas" means Qantas?

Thanks, kevin, I was thinking that was an odd line of succession... :/

kevin and @Him, thanks!


text:
Why is the DC10/MD11/TriStar vertical stabilizer noticeably smaller than other contemporary aircraft?
Why is it that these planes had vertical stabilisers that were noticeably smaller in proportion the aircraft itself to other aircraft? I understand that they had the engines mounted on the tail but couldn't the surfaces have been made larger? Doesn't this make it more difficult to control during flight as the surface is way smaller? Maybe is had some other mechanism in the engine that could compensate for the smaller area of the vertical fin?
I've added some images of the DC10, the TriStar, Boeing 767 and A310 which are aircraft that are sort of comparable in era and age and the most interesting one is the TriStar which so small:

Two characteristics drive vertical tail size in airliners:

Critical engine out: Here the tail needs to compensate the yawing moment of the live engine on one wing.
Yaw damping: This is aided by a yaw damper, but enough damping must remain for the case of a failed damper. Especially the dutch roll eigenmode is driving tail size.

On the other hand the tail should not be bigger than absolutely necessary to keep drag and weight down. On the Tristar, the big intake added so much tail area that a relatively small tail was sufficient. Note that the engine intake on the Tristar is much longer than on the DC-10 because of the S-duct. The flow must be brought down to the engine without internal flow separation, which is made easier when the intake is longer.
Since in the engine-out case only one third of total thrust creates the yawing moment which the tail needs to overcome compared to one half of the thrust in case of the A310, the vertical tails of both the Tristar and DC-10 are smaller relative to that of the A310. The A310 is a shortened version of the A300, so the reduction in fuselage length made an especially large vertical tail necessary. If you look at the vertical tails of the MD-80, the Il-62 or the VC10, they are even smaller since very little yawing moment needs to be compensated in the engine-out case. Adding to this, the T-tail configuration makes the existing tail more effective due to the endplate effect (PDF!) of the horizontal tail on those three types.

Vickers VC10 (left) and Tristar (right) tankers in flight (picture source).

Am I seeing things, or is that VC10 reversing its outboard engines in flight?

Does the length of the lever arm from the CoG have any effect? Is it relevant in this case?

@ymb1: Ah, makes sense.

Doesn't this make it more difficult to control during flight as the surface is way smaller?

Design
Rudder authority becomes most important (arguably) when one wing-mounted engine fails or is shut down.
To combat this on the DC10

The rudder takes up a large proportion of the area of the fin
The rudder has dual articulation - there are two vertical hinges.
This gives the rudder greater effectiveness.
The wing-mounted engines are mounted closer to the fuselage.
This reduces the yawing effect of a dead engine.
The wing-mounted engines provide only 1/3rd of the thrust instead of
half as with a 2-engine aircraft,
so the effect of one engine out is less.

The 747SP is another aircraft with dual articulation in the rudder.
Operating Procedures
It may be the case that the DC10 operating procedures had lower limits than some other aircraft for cross-wind on landing.

Perhaps an addition to this answer: the wing-mounted engines provide only 1/3rd of the thrust instead of half as with a 2-engine aircraft, so the effect of one engine out is arguably less.

@sanchises That's not really arguable at all. - haha - It's definitely less.

@sanchises: Excellent point, thanks. Added to answer as suggested.

Also, the DC-10's big tail-engine air intake adds to the effective area of the vertical stabiliser.


text:
What is "established" on an RNAV approach?
I was looking at the RNAV (GPS) RWY 6 at 12J.  Thinking about my descent profile, I wondered when I could start my descent from my transition altitude if I was told to maintain an altitude until established.
For example, if the controller told me to "maintain 4000 until established," would I start descending after crossing the IAF and tear-dropping into the hold?  Would I start descending on the inbound leg?  Or, would I start descending after leaving the IAF inbound on the approach?
Edit:
This question assumes that a hold-in-lieu-of-procedure turn is required.  The question, really, is if a controller says "maintain 4000 until established," when can you start descending?  Can you descend as soon as you cross the IAF?

You might want to change the title of your question to match the body (or vice versa). The title asks what "established" means, but the body asks when you can start a descent.

NOTE: This answer is specific to the United States as the question uses an example approach procedure from an airport in the United States. Procedures may vary for other nations. Consult the respective Aeronautical Information Publication for specific guidance.
As long as you hear the magic words "cleared  approach," you can begin your descent as soon as you cross the IAF the first time (while beginning your teardrop maneuver) and continue to descend to the minimum altitude while in the circuit, which is at or above 2000 for this particular procedure.
Note that you MUST be on a published segment of the approach before making your descent, otherwise terrain clearance is not assured.
The controller will expect you to proceed from the IF to the FAF after the first circuit is complete (a course reversal maneuver, if executed counts as the first circuit). You must advise the controller if additional circuits are required in order to lose altitude. Also, if the controller clears you for a "straight-in" approach and doing so would result in an excessive descent rate, it is incumbent upon you as the pilot to advise the controller that you are unable.
Reference: FAA JO 7110-615, Approach Clearance

AIM 5-4-5 says

Once cleared for the approach, pilots may
descend in the TAA sector to the minimum altitude
depicted within the defined area/subdivision, unless
instructed otherwise by air traffic control. Pilots
should plan their descent within the TAA to permit a
normal descent from the IF/IAF to the FAF.

The diagram (in the 2019 version on p. 5-4-12) shows sector altitudes without the notation NoPT, implying that the paragraph just quoted applies to them as well. You don't have to wait to cross the IAF before you can descend in this case to 2000 or 2300 depending on the sector you're arriving from.
See also the discussion here.


text:
How are wingtips attached to the aerofoil while maintaining structural rigidity?
I've been trying to research various methods on the joints between the winglet and the wingtip out of interest. I'm aware that the airfoil is attached to the underbody of the fuselage by means of a butt joint, but with companies like Aviation Partner who offer options such as BBJ's, how are these winglets attached?

This is pedantic, but isn't the wingLET attached to the wingTIP? (Hey guys, I've got this winglet, now let's build a plane...)

Is a butt joint some kind of aerostructures term I'm not familiar with? Typically the wing spar extends into the fuselage quite a bit where it is structurally connected to the fuselage bulkhead(s) and the other wing...

There are a number of winglets available in market for retrofitting and their method of attachment will also vary. Boeing details the procedure for attaching a winglet to their BBJ and it is quite instructive. According to the article the main issue is modifying the wing to accommodate the winglet, rather than the attachment itself.

The more time-consuming part of the retrofit is installation of the
  wing modification to accommodate the winglet.

For this particular aircraft, the following steps are given:

Removal and replacement of the outboard upper and lower skin panels
Removal and replacement of rib 25, which is third from the outermost rib
Installation of stiffeners across rib 25.
Cutting of the closure rib (rib 27) and trimming of the two spars
Installation of the new center section of rib 27 and the
  new winglet attach fitting
Installation of the spar attach fittings
Installation of the aft-position light.

which is followed by installation of the winglet.

Installation of winglet, from Boeing Aero.

How many ribs would say a B737-800 have without a winglet?

@PravinPradeep it says that rib 27 is already there, they just modify it, so there are still 27 ribs.

@PravinPradeep - sounds like a whole new question - you should ask it!


text:
Can you feel passing through the sound barrier in an F-16?
Is it possible for the pilot to feel passing through the sound barrier in an F-16?
What about other modern aircraft, will you feel anything? Do you need to go back to really dated designs before you can notice this, is it a problem of the past, or did the documentaries I watched which discussed this problem grossly exaggerate it?

FWIW, the sound "barrier" is not so much a thing that an aircraft encounters in flight as it was an obstacle that aircraft designers faced before they fully understood the nature of supersonic flows, and learned how to build aircraft that were capable of controlled, supersonic flight.

In an aircraft designed to go supersonic, it's an absolute non-event, and one is only aware of it by observing the instruments, and noting diminished control authority-- slower roll rate, etc. At least, that was my experience in the T-38, and according to every account I've read.
If the aircraft is NOT designed to go supersonic, then the experience can be quite different, although that mostly comes from loss of control at high Mach numbers (starting in the 0.9X range, as best I recall), rather than anything that occurs right at Mach 1.00.

I've heard of the P-38s going transonic in dives and not being able to recover until they can get into denser air.

Thank you for your answer. Are you saying that there is a sudden jump (or rather, a fall) in control authority once passing mach 1? So in the T-38, going from mach 0.9 to mach 1.1 results in a much greater loss of control authority than going from mach 0.7 to mach 0.9, in the same aircraft?

Generally, the T-38 had an amazing roll rate, like over 360 degrees per second. Supersonic, though, it was way less, maybe a quarter or an eighth of that. Don't remember exactly when it dropped off; it was NOT a noticeable thing at Mach 1; you noticed a twitch in the airspeed & altitude indications, but only that. Then you try the aileron roll, and it seems to take forever. We didn't demo that at .90, .95, etc for comparison, unfortunately.

@BillDOe:  There is no recorded case of the P-38 going faster than sound.  There were problems in high speed dives that cause loss of control, but they were not caused by the P-38 breaking the sound barrier.  There were P-38s that broke up in dives.  The "bang" from the breakup was (falsely) interpreted as a sonic boom.

I wonder what it would feel like in an A320...

@Cloud I think that the answer explains that

@JRE He said transonic, not supersonic. The loss of control authority on P-38s in high speed dives was related to parts of the airflow around it becoming supersonic (but I agree definitely not the plane as a whole), see https://en.wikipedia.org/wiki/Mach_tuck

Doesn't it get a little quieter in the cockpit at supersonic speeds?

Yeah, that's what I was going to say. Would the wind noise decrease? Or is there just no audible wind noise in the first place?

@JRE, the term I used was transonic, not supersonic. I've always understood the former to mean that some parts of the airflow are forced to move at supersonic velocities, while the plane itself is still subsonic. This then causes compressibility issues that deflect normal airflow away from the plane's control surfaces.

Does anyone have good detailed data recordings from a modern jet transitioning into supersonic speeds to add to the answer? Scientific measurement kind of data I mean. [I spent some time in the back of an F-18 on research flights, and could hear/feel subtle differences based on speed, but given that I can hear far lower than average frequencies I'm probably a terrible data point...]

You can watch for yourself 4 minutes in on this video.  Can't tell at all, so much so that they have to let people know with a big sign.  Once the issues of buffeting during the transition were fixed in the design of supersonic aircraft, pretty anticlimactic.

here exactly: https://youtu.be/YeEB2Lxbfa4?t=229

In the F-18, you experienced a very slight "tuck", which was basically the nose of the aircraft pitching around the horizontal axis very slightly. But, it was extremely minor, and in order to notice it you needed to be in smooth air, and paying close attention.
In 99% of transitions through "the number", you were so focused on other stuff you noticed nothing.

Very interesting answer. Perhaps you could add the source for this? It seems implied that you either are or were an F/A-18 pilot, is this true? Or it this second hand information from someone else?

I am the source, so indeed it is hard. I flew F-18's for the Australian Air Force in a past life.

Thank you for your comment, and answer.


text:
Is an F-16 able to fire any of its weapons while on the ground?
I can recall at least one Hollywood movie, where this takes place (although not an F-16 in the movie).
Is this at all possible? Can the F-16 fire its M61 while taxiing on the ground? What about other weapons?
Note that I'm not even sure myself what good this would do or what the point would be. I'm just curious.
I can imagine the bullets would not even be able to hit a target on the ground, perhaps pass over or below the target. Does the M61 even point perfectly parallel to the ground on an F-16? At least it would change its elevation by some degrees up and down when taxiing, braking, etc. I suppose I could imagine helmet cuing of a low-flying target with an AMRAAM while the aircraft is still trying to take off. Since it has happened before that an airbase is taken by surprise, perhaps this question is not as silly as I at first thought. However, I'm under no illusion that an AMRAAM would be able to engage any target when fired from an aircraft on the ground---doesn't the missile need some time and space to drop from the rail and fire its engine?
I'd love to be told that I'm wrong though.
Nevertheless, I'm curious if the aircraft is designed to prevent firing its weapons while the landing gear is out, or a similar safety feature. If so, can it be overridden? Specifically, what, if anything, prevents an F-16 from firing its M61 while on the ground?
As a bonus question, has this by accident ever occurred, that an aircraft (F-16, or other) fired its weapons while stationary, or taxiing around at an airport?

Back in the days of .50 cal, they had to fire the weapons on the ground to adjust/verify convergence and sighting.  Still the case?  Dunno.

Yes, the F-16 can fire on the ground, and it has happened as an accident

A maintenance worker accidentally fired a 20mm Vulcan cannon from an F-16 jet he was working on at Belgium's Florennes Air Force Base earlier this week, destroying another F-16 while damaging another aircraft nearby, according to multiple media reports.
A worker apparently opened fire across the flight line while performing maintenance on Oct. 11, according to Avio News. The F-16 that was racked by cannon fire quickly caught fire and exploded, since it was loaded with fuel and being readied for an afternoon sortie, according to Scramble Magazine.

https://taskandpurpose.com/f16-destroyed-belgium

It should be added that "by default" it is not possible. The WoW system (Weight-On-Wheels) must be deactivated/overridden

There is no evidence in the news article that the technician had to disable a system to fire the weapon.  In fact, it seems to have been done entirely inadvertently.   Do you have any source to backup the assertion that WoW inhibits weapons?

Without knowing any more about it, I'd bet the technician was performing a "before loading" function check without knowing the gun had already been loaded, i.e. communication breakdown and/or checklist skip.

@abelenky Also see this duplicate https://aviation.stackexchange.com/questions/56064/what-measures-prevent-a-military-aircrafts-weapons-from-being-fired-on-the-grou

@abelenky – I wouldn’t take the media’s omission of detail around failsafe measures to indicate that there aren’t any.  (They aren’t known for being authorities on matters like this)  It is extremely unlikely that the F-16’s weapons systems are not safed on the ground through WoW, and I would ask for evidence to be convinced otherwise.

@MichaelHall: Evidence is that the gun fired, and destroyed another aircraft.  If the WoW inhibited gun-firing, that would not have happened

Not necessarily.  I have decades of experience reading and writing military mishap reports.  It is a lesser stretch of the imagination to presume that a maintenance function temporarily disabled the WoW protection than to assume that it was never designed in.  Again, the media rarely gets these details correct, and they wouldn't have access to the mishap investigation report.

To my point about trusting media knowledge and accuracy concerning aviation matters, I present the following evidence:  https://www.youtube.com/watch?v=n6aclrHYTh8

Adding in late here: "Can an F-16 fire its weapons on the ground?" Yes, given one of 2 conditions - 1 purposely activating the WoW switch by a crewman in the nose wheel well. 2. Malfunction of the WoW switch and/or associated wiring. (source - my 16 years as a USAF armament specialist, 9 years on F-16)